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Project

Shape coexistence in the nickel (Z=28) and mercury (Z=80) regions probed through decay studies

In the last 60 years, the understanding of shape coexistence, a phenomenon when low-lying nuclear structures are exhibiting different deformations, evolved from being a rarity which occurs only in selected nuclei, to an almost universal property throughout the whole chart of nuclides. There are many fingerprints pointing toward shape coexistence in the nucleus, with the spectroscopic quadrupole moment being the most important. This phenomenon can be also studied by analyzing the decay pattern or the conversion coefficients. The former might show a structure difference with the transitions hindrance while the latter might suggest an existence of E0 transition, which points to the mixing of configurations of different deformations.

In this thesis, shape coexistence is studied in two regions of the chart of nuclides, neutron-rich nickel isotopes around N=40 and neutron-deficient mercury isotopes around N=104, by the means of β-decay, performed at the ISOLDE facility at CERN.

The first experiment was focused on study of the A=66 chain, populated by the β- decay of 66Mn produced in fission of a UCx target, induced by 1.4 GeV protons. The isotopes of interest were selectively ionized, mass separated and delivered to the Leuven Decay Station, where the emitted β and γ radiation was detected by the three plastic scintillators and two MiniBall cluster germanium detectors, respectively. The analysis of the collected data result in decay schemes of 65,66Fe, 66Co and 66Ni and the determination of γ-branching ratios and log(ft) values. The comparison with the Monte Carlo shell model calculations allowed to explain the selective β-feeding of the 0+ and 2+ states in 66Ni as being linked to the difference in deformation. The calculations also reproduced other observables in the A=66 chain, including the selective β-feeding of the 2.9 MeV level in 66Fe and the order of low-lying states in 66Co.

The second experiment was dedicated to the study of excited states in 182,184,186Hg populated in the β+-decay/electron capture of 182,184,186Tl. The pure beam produced by 1.4-GeV-proton-induced-spallation of UCx target, selective laser ionization and mass separation was delivered to the ISOLDE Decay Station, where the radiation was detected by five High-Purity Germanium clovers and a segmented, liquid-ethanol cooled silicon detector, the SPEDE spectrometer. The collected data allowed to confirmed and extend the decay scheme and to increase the precision of the spectroscopic information. In addition, several new E0 transitions have been identified pointing to the mixing of the 4+ and higher-spin states. The data was compared to four theoretical models: Interacting Boson Model with Configuration Mixing, General Bohr Hamiltonian, Beyond Mean-Field and Symmetry-Conserving Configuration Mixing method, and the qualitative agreement has been reached, however, the quantitative description is still lacking.

Date:1 Sep 2016 →  28 May 2021
Keywords:mercury, shape coexistence, nuclear spectroscopy
Disciplines:Instructional sciences, Nuclear physics, Condensed matter physics and nanophysics, Classical physics, Elementary particle and high energy physics, Other physical sciences, Applied mathematics in specific fields, Quantum physics
Project type:PhD project